This invention relates to electronic digital data communication area.
Conventional method of transmitting digital data to a receiver is to modulate a sine wave. Sine wave has three parameters: amplitude, frequency, and phase. One of these parameters or a combination of them are modified to represent a digital data. For example to transmit two digital bits using frequency variation the following technique can be used. Two binary bits {0, 1} can be combined in 2**2=4 ways. They are {00, 01, 10, 11}. The pattern 00 can be represented by a sine wave with 100 Hz frequency, 0.1 by 200 Hz, 10 by 300 Hz, and 11 by 400 Hz. If the receiver detects 300 Hz sine wave signal then it knows that the transmitted bit pattern is 10. Observe that to transmit two bits of data at a time four symbols are required, and the receiver will have to search over all of them.
In the above example, each sine wave with the above frequencies is a symbol for each of the digital patterns. To transmit two bits it takes four symbols. Similarly, to transmit 8 bits it will take 2**8=256 symbols. Therefore to detect an 8-bit pattern the receiver mill have to search 256 signals or symbols to find the transmitted bit pattern. To add one more bits to the transmitted data, for example from 8 bits to 9 bits, the symbol space will increase from 256 to 2**9=512. Thus the symbol space becomes double for every additional bits. This approach to symbol space puts significant restrictions on the technology for transmitting at a high data rate.
In the conventional approach the information content in the symbol is very low. Only one of the parameters of the sine wave differs from symbol to symbol, and the parameter remains constant over the symbol time. In Quadrature Amplitude Modulation (QAM) method two sine waves are used with two different amplitudes. QAM is also equivalent to a symbol with both amplitude and phase modulation. In QAM only two parameters are different in each symbol.
These are very inefficient use of the symbol time. A lot more information can be transmitted over the same symbol time by properly designing the symbols and still maintaining the same signal bandwidth.
The invention describes a concept of increasing the information content of symbols used to transmit digital data. Each bit of the digital data is represented by an unique function. All these unique functions are added together to create a new function. This new function is the symbol for the complete digital data that will be transmitted to a receiver over a communication medium.
The decoding process essentially is the process of finding binary solutions of a set of linear equations. These linear equations represent the symbol function values in terms of the bit function values. The unknown variables of these equations represent the values of the bits of the digital data.
Coding process described above gives the capability to increase the information content of the symbol. Therefore more bits can be represented by a symbol, and more bits can be transmitted in the same symbol time. By reducing the frequency contents of the bit functions, the bandwidth requirements can also be reduced. This approach of coding and decoding will enable higher communication rate for voice band telephone modems.
In the description several methods for coding and decoding of digital data are presented. These methods are used to illustrate the concept behind the coding/decoding approach presented here. What is invented is not these methods but the concept behind these methods. In coding, the invented concept is to increase the information content of the symbol, by assigning information to each bit of the digital data and then combining these information items to representing the entire digital data. In decoding the concept is to express the combined information in terms of the known bit information and unknown bit variables. The recovery of the binary data is then equivalent to looking for binary solutions of the unknown bit variables.